Catalyst systems which are solutions of one or more catalyst components (e.g. a transition metal compound and optionally a cocatalyst) are known in the filed as homogeneous catalyst systems. Homogeneous systems are used as liquids in the polymerisation process. Such systems have in general a satisfactory catalytic activity, but their problem has been that the polymer thus produced has a poor morphology (e.g. the end polymer is in a form of a fluff having a low bulk density). As a consequence, operation of slurry and gas phase reactors using a homogeneous catalyst system cause problems in practice as i.a. fouling of the reactor can occur.
To overcome the problems of the homogeneous systems in a non-solution process the catalyst components have been supported, e.g. their solution impregnated, on porous organic or inorganic support material, e.g. silica. These supported systems, known as heterogeneous catalyst systems, can additionally be prepolymerised in order to further immobilise and stabilise the catalyst components.
However, also supported and optionally prepolymerised systems have problems. It is difficult to get an even distribution of the catalyst components in the porous carrier material; and leaching of the catalyst components from the support can occur. Such drawbacks lead to an unsatisfactory polymerisation behaviour of the catalyst, and as a result the morphology of the polymer product thus obtained is also poor. Furthermore, the uneven distribution of the catalyst components in the support material can have an adverse influence on the fragmentation behaviour of the support material during the polymerisation step.
The support can also have an adverse effect on the activity of the catalyst, on its polymerisation behaviour and on the properties of the end polymer.
Accordingly, prior art proposes a variety of different means for achieving an even distribution of the catalyst component(s) into the pores of a support material, and in particular various impregnation methods have been suggested: The support material can be slurried in the solution of the catalyst components, or said solution is added to the silica in a volume which is equal or less than the total pore volume of the used silica (see e.g. WO 95 12622 of Borealis). WO 97 17136 of Mobil oil discloses a method wherein Mg-treated silica is slurried to an aliphatic solvent, e.g. isopentane and a toluene solution of a metallocene and aluminoxane is added to the slurry. The catalyst components are allowed to impregnate in the formed mixture and finally the solvents of the mixture are removed by evaporation. EP 295 312 of Mitsui discloses a method wherein aluminoxane in a first solvent is precipitated by adding a second solvent wherein aluminoxane is insoluble, in the presence of a carrier to deposit the precipitated aluminoxane on the carrier.
WO 97 02297 of Exxon describes a variation of a conventional slurry prepolymerisation method wherein the amount of the solvent used for the supported catalyst particles during the prepolymerisation step is reduced to a volume equal or less than the total pore volume of the supported catalyst. According to said document said prepolymerisation conditions has to be adjusted to avoid any clumping problems due to the condensation of the solvent (which may lead to the migration of the catalyst components already encountered with the prior art slurry prepolymerisation method).
Due to the complexity of the catalyst systems, the need still exists to develop further catalyst systems and preparation methods thereof which overcome the problems of the prior art practice.